The present invention relates to heat-insulating structures used for engines, etc.
Metallic products, such as engine parts, which are exposed to high temperature gas are provided with a heat-insulating layer on a surface of the metallic base material thereof to reduce heat transfer from the high temperature gas to the base material. For example, Japanese Patent Publication No. 2009-243352 discloses that a heat-insulating film containing hollow ceramic beads is provided on a surface of an engine part facing a combustion chamber. Japanese Patent Publication No. H05-58760 discloses that surfaces of hollow siliceous spheres are coated with fine alumina particles; that the coated spheres are press-molded; and that the resulting molded body is sintered to obtain a heat-insulating material. Japanese Patent Publication No. 2005-146925 discloses that protrusions and grooves are formed in a surface of an engine cylinder head facing a combustion chamber, and that the grooves are filled with a zirconia-based, low-heat-conductivity material to increase heat resistance of the cylinder head.
To increase fuel economy of a vehicle, attempts are being made to reduce the weight of the vehicle body, improve the thermal efficiency of the engine, reduce mechanical resistance, reduce electrical load, and collect and use exhaust energy, etc. Here, it is known that in theory, the thermal efficiency of the engine increases as a geometric compression ratio is increased, or as an excess air ratio of an operative gas is increased (i.e., as a specific heat ratio is increased). However, in reality, the cooling loss (i.e., energy dissipated to the outside as heat) increases as the compression ratio is increased, or the excess air ratio is increased. Therefore, there is a limitation in improving the thermal efficiency by increasing the compression ratio or the excess air ratio.
Specifically, the cooling loss depends on a coefficient of heat transfer from the operative gas to the engine combustion chamber wall, a heating surface area of the wall, and a difference between a gas temperature and a wall temperature. The heat-transfer coefficient is a function of a gas pressure and a gas temperature. Thus, if the gas pressure and the gas temperature are increased due to an increase in compression ratio and excess air ratio, it leads to an increase in heat-transfer coefficient and results in greater cooling loss. A difference between the wall temperature and the gas temperature is increased as well, which also results in greater cooling loss. Thus, although setting the compression ratio to a very high compression ratio (e.g., 20 or more) results in a higher expansion ratio, and is effective in reducing exhaust loss, it is difficult to set the compression ratio to a very high compression ratio for reasons of greater cooling loss as described above.
Alternatively, the efficiency of the engine may be increased (or fuel economy may be improved) by collecting exhaust energy without significantly increasing the compression ratio. However, in this case too, the greater the cooling loss is, the smaller the exhaust energy becomes. Therefore, similarly to the case of increasing the compression ratio, it is important to reduce the cooling loss.